This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9903920-8 filed in Sweden on Oct. 29, 1999; the entire content of which is hereby incorporated by reference.
This invention relates to the technical field of antennas, and more specifically to dual-polarised antennas.
Historically, the most common diversity technique, especially in mobile communications, has been space diversity. In this diversity technique two or more antennas are placed apart from each other, separated by a distance that is a function of the wavelength the antennas should receive or transmit. On uplink the incoming signals received at each antenna are combined in an optimum way so as to maximise the quality of the resulting signal.
Owing to economical as well as aesthetical reasons, base stations are nowadays increasingly equipped with a single dual-polarised antenna, or one such antenna for each sector or frequency band. Such an antenna must provide two at least mainly orthogonal polarisation directions in order to enable the use of diversity techniques. This is possible, as orthogonally polarised waves are essentially uncorrelated in a multipath environment.
Good transmission and reception characteristics are obviously important in antennas. For dual-polarised antennas, these characteristics are among other things affected by isolation between the antenna""s feed ports, and by the farfield pattern characteristics. Good isolation between feed ports is much more difficult to obtain in a dual-polarised antenna than in a pair of space diversity antennas as the latter are located a distance apart from each other.
The isolation problem is easier to describe if an antenna is considered as analogous to a four-port. Two of the four ports represent actual transmission line feed ports; one for each of the two desired orthogonal polarisations. The other two ports are virtual ports representing, on transmit, radiated power in each of the two orthogonal polarisations, integrated over an arbitrary sphere enclosing the antenna. Thus, on transmit, the antenna has two input ports (feed ports) and two output ports (radiated power). Similarly on receive the antenna has two input ports (received power) and two output ports (feed ports). Note the the same feed ports are used for both transmission and reception. The antenna, and its four-port representation, is reciprocal.
The scattering parameters of a four-port are often represented by the S-matrix, a four-by-four matrix. The S-matrix for an ideal dual-polarised antenna has four non-zero values all of unit magnitude. These values represent the forward and backward coupling between corresponding input and output ports.
Isolation between ports is never perfect in practice. This leads to a certain degree of mutual coupling. From a system point of view, the study of mutual coupling effects can be limited to two categories: isolation between the feed ports and coupling between feed port and undesired output port. Isolation between feed ports is primarily a problem on tranmit while coupling between feed ports and undesired output ports is primarily a problem on receive.
Isolation between feed ports is of primary importance in the transmit direction, i.e. in the downlink band for mobile telecommunication. In a base station antenna, transmitted power is many orders of magnitude greater than received power. It is therefore important to stop transmitted signals from leaking into the received signal paths. This is achieved with filters, and the worse the mutual coupling between feed ports the worse the leakage and the better the filters have to be. Better filters, which give better suppression, are as a rule more expensive and unwieldy. On the other hand, reduced mutual coupling between feed ports enables the use of simpler and less expensive filters.
Mutual coupling between feed ports can also cause problems when the transmitted signal from one port travels xe2x80x9cbackwardsxe2x80x9d via the other transmit branch. This may give rise to intermodulation effects and spurious radiation, both in the uplink and downlink frequency bands.
Coupling between a feed port and undesirable output port will result in polarisation impurities. On receive the impurities may increase the correlation between the received signals, which in turn diminishes the diversity gain potential. The similarity between the angular power distribution of the radiation patterns for the two polarisations will also affect the attainable diversity gain. In general, the more alike the radiation patterns of the two polarisations are, the better the antenna will be from a polarisation diversity point of view. Furthermore, equal patterns for both polarisations are important on both uplink and downlink in order to cover the same angular and radial region.
There are a number of factors that adversely affect the polarisation purity and the mutual coupling of an antenna. The finite size of the antenna will give rise to both of the above owing to edge and corner diffraction. The antenna may consist of one, two or more primary radiators (antenna elements). Dual-polarised primary radiators also generate mutual coupling and polarisation impurities. This is owing to asymmetries in both the radiating elements and the feed network. Mutual coupling can also be caused by the proximity of the element feed points.
Several attempts have been made to overcome these problems with dual-polarised antenna elements. Some of these attempts will be described below.
A common type of dual-polarised antenna element is the aperture-coupled microstrip patch antenna. By feeding a patch using orthogonal slots (apertures) the antenna is made to simultaneously radiate and/or receive two orthogonally polarised waves. Similar characteristics can be achieved by using probe-fed patches, but the aperture-coupled patch is superior to the probe-fed patch from a bandwidth, passive intermodulation and manufacturing point of view and is the dominant type in use today for communication applications.
U.S. Pat. No. 4,903,033 presents a dual-polarised antenna that involves an air bridge to accomplish a symmetric feed arrangement in both of the polarisation branches, while Sanford, J. R. and Tengs, A.: xe2x80x9cA Two Substrate Dual-Polarised Aperture-Coupled Patchxe2x80x9d, Proceedings 1996 IEEE Antennas Propagation Society Symposium, present an antenna where symmetry is achieved by placing the feed networks on opposite sides of a dielectric substrate, with the feed slots de-embedded in the centre of the substrate. Both solutions involve feed network layouts that make the dual-polarised patch element more complex than a single polarised element. Furthermore, neither of the two solutions is truly symmetric. Mutual coupling, primarily between the two crossing feed arms, will introduce asymmetries in the excitation current. These asymmetries can only be compensated for at one or a few frequencies, if at all.
U.S. Pat. No. 5,045,862 presents a microstrip array arrangement useful for reception with a high degree of symmetry. This single layer arrangement consists of interconnected etched square patch elements and filters. These elements and filters make up a square periodic grid, in two orthogonal directions. Each patch is connected to its four neighbouring patches by symmetric feed lines extending from the centre of the four sides of the patch element. While this arrangement is truly symmetric, it is not useful for communication applications for a number of reasons. For example, radiation may occur from feed lines, filters, and matching stubs when using one side of the dielectric layer, and the coupling between polarisations is not suppressed by the feed arrangement.
WO 98/49741 proposes a compact and simple feed solution, see FIG. 1. A single layer feed-network design is utilised with a combination of a symmetrical and an asymmetrical feed arrangement. This solution can only be used to obtain a two-port antenna element with good isolation properties at one or a few frequencies. Nor does the design allow for simultaneous symmetrical slot feed for both polarisation ports.
For the reasons stated above it is desired to find a dual-polarised antenna element with improved transmission and reception characteristics.
The present invention aims to solve the problem of how to improve transmission and reception characteristics in dual-polarised antennas.
One object of the present invention is to provide a dual-polarised antenna element for which the transmission and reception characteristics are improved owing mainly to the layout of the feed network.
Another object of the present invention is to provide a layout of a feed network for a dual-polarised antenna in order to reduce or cancel undesired effects from one part of the network to another and vice versa.
Yet another object is to provide a method for feeding current to two orthogonal polarisations in a dual-polarised antenna element.
Still another object is to provide a method for obtaining a dual-polarised antenna element of the above-mentioned kind.
According to the present invention, there is provided a dual-polarised antenna where the effect of the signals feeding one polarisation, on the other polarisation, is cancelled owing to the layout of the lines through which the signals are fed.
According to the present invention, there is provided a method for feeding current to two orthogonal polarisations in a dual-polarised antenna element so that the effect of the current on the other polarisation is cancelled owing to the layout of the lines through which the signals are fed.
According to the present invention, there is also provided a method for obtaining a dual-polarised antenna element of the above-mentioned kind.
The apparatus according to the invention is defined in claim 1.
Preferred embodiments of the apparatus according to the invention are defined in claims 2-24.
The first method according to the invention is defined in claim 25.
The second method according to the invention is defined in claim 26.
Preferred embodiments of the second method according to the invention are defined in claims 27-35.
An advantage with the present solution to the problem is that the port isolation in the dual-polarised antenna is improved.
Another advantage with the present solution to the problem is that the similarity of the dual-polarised antenna""s patterns of the two polarisations is increased.
The invention will now be described in more detail with the aid of the description of the embodiment and with reference to the drawings.